The present invention relates to a heat pump, and more particularly to a heat pump employing a capillary tube to control refrigerant flow.
In a heat pump employing a simple capillary tube to control the flow of refrigerant, the range over which control is possible is limited, since the capillary tube is not adjustable. Accordingly, the idea has been conceived of increasing the range over which flow control can be performed by providing adjustable means for cooling the refrigerant as it flows through the capillary tube. If the two-phase refrigerant passing through the capillary tube is cooled, the amount of vapor in the refrigerant will be decreased by the cooling and accordingly the mass flow rate of the refrigerant will be increased. Therefore, by controlling the amount of cooling of the capillary tubes, the range of control of the flow through the capillary tubes can be considerably increased.
Japanese Laid-Open Patent Applications Nos. 58-28960 and 58-28961 disclose heat pumps in which the majority of the refrigerant is passed through a capillary tube, while a small portion of the refrigerant is passed through a controllable expansion valve and then passed through cooling pipes surrounding the capillary tube, thereby cooling the refrigerant passing through the capillary tube. The refrigerant passing through the capillary tube and the portion passing through the expansion valve are combined downstream of the capillary tube and together pass into the evaporator of the heat pump.
In those heat pumps, while the range of control of refrigerant flow is increased, it is not possible to achieve optimal refrigerant flow except in an intermediate operating range. Namely, since refrigerant always flows through the capillary tube even when the expansion valve is completely closed, more than the optimal amount of refrigerant will flow through the capillary tube when the cooling or heating load is low unless the diameter of the capillary tube is made extremely small. On the other hand, even if the expansion valve is fully opened, the amount of refrigerant which will flow through the capillary tube will be less than the optimal amount when the cooling load is very large unless the capillary tube is made large. Thus, with a single capillary tube, it is impossible to size it so that it will supply the optimal amount of refrigerant at both minimum and maximum load.